<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
    <title>WebGPU Compute Shaders - Histogram, slow</title>
    <style>
      @import url(resources/webgpu-lesson.css);
      canvas {
        display: block;
        max-width: 256px;
        border: 1px solid #888;
        background-color: #333;
      }
    </style>
  </head>
  <body>
  </body>
  <script type="module">
import TimingHelper from './resources/js/timing-helper.js';
// see https://webgpufundamentals.org/webgpu/lessons/webgpu-utils.html#webgpu-utils
import {
  loadImageBitmap,
  createTextureFromSource,
} from '../3rdparty/webgpu-utils-1.x.module.js';

async function main() {
  const adapter = await navigator.gpu?.requestAdapter();
  const canTimestamp = adapter?.features.has('timestamp-query');
  const device = await adapter?.requestDevice({
    requiredFeatures: [
      ...(canTimestamp ? ['timestamp-query'] : []),
    ],
  });
  if (!device) {
    fail('need a browser that supports WebGPU');
    return;
  }

  const timingHelper = new TimingHelper(device);

  const module = device.createShaderModule({
    label: 'histogram shader',
    code: /* wgsl */ `
      @group(0) @binding(0) var<storage, read_write> bins: array<vec4u>;
      @group(0) @binding(1) var ourTexture: texture_2d<f32>;

      // from: https://www.w3.org/WAI/GL/wiki/Relative_luminance
      const kSRGBLuminanceFactors = vec3f(0.2126, 0.7152, 0.0722);
      fn srgbLuminance(color: vec3f) -> f32 {
        return saturate(dot(color, kSRGBLuminanceFactors));
      }

      @compute @workgroup_size(1, 1, 1) fn cs() {
        let size = textureDimensions(ourTexture, 0);
        let numBins = f32(arrayLength(&bins));
        let lastBinIndex = u32(numBins - 1);
        for (var y = 0u; y < size.y; y++) {
          for (var x = 0u; x < size.x; x++) {
            let position = vec2u(x, y);
            var channels = textureLoad(ourTexture, position, 0);
            channels.w = srgbLuminance(channels.rgb);
            for (var ch = 0; ch < 4; ch++) {
              let v = channels[ch];
              let bin = min(u32(v * numBins), lastBinIndex);
              bins[bin][ch] += 1;
            }
          }
        }
      }
    `,
  });

  const pipeline = device.createComputePipeline({
    label: 'histogram',
    layout: 'auto',
    compute: {
      module,
    },
  });

  const imgBitmap = await loadImageBitmap('resources/images/pexels-chevanon-photography-1108099.jpg'); /* webgpufundamentals: url */
  const texture = createTextureFromSource(device, imgBitmap);

  const histogramBuffer = device.createBuffer({
    size: 256 * 4 * 4, // 256 entries * 4 (rgba) * 4 bytes per (u32)
    usage: GPUBufferUsage.STORAGE | GPUBufferUsage.COPY_SRC,
  });

  const resultBuffer = device.createBuffer({
    size: histogramBuffer.size,
    usage: GPUBufferUsage.COPY_DST | GPUBufferUsage.MAP_READ,
  });

  const bindGroup = device.createBindGroup({
    label: 'histogram bindGroup',
    layout: pipeline.getBindGroupLayout(0),
    entries: [
      { binding: 0, resource: { buffer: histogramBuffer }},
      { binding: 1, resource: texture.createView() },
    ],
  });

  const encoder = device.createCommandEncoder({
    label: 'histogram encoder',
  });
  {
    const pass = timingHelper.beginComputePass(encoder);
    pass.setPipeline(pipeline);
    pass.setBindGroup(0, bindGroup);
    pass.dispatchWorkgroups(1);
    pass.end();
  }

  encoder.copyBufferToBuffer(histogramBuffer, 0, resultBuffer, 0, resultBuffer.size);

  const commandBuffer = encoder.finish();
  device.queue.submit([commandBuffer]);

  timingHelper.getResult().then(duration => {
    console.log(`duration: ${duration}ns`);
  });

  await resultBuffer.mapAsync(GPUMapMode.READ);
  const histogram = new Uint32Array(resultBuffer.getMappedRange());

  showImageBitmap(imgBitmap);

  // draw the red, green, and blue channels
  const numEntries = texture.width * texture.height;
  drawHistogram(histogram, numEntries, [0, 1, 2]);

  // draw the luminosity channel
  drawHistogram(histogram, numEntries, [3]);
}

function drawHistogram(histogram, numEntries, channels, height = 100) {
  // find the highest value for each channel
  const numBins = histogram.length / 4;
  const max = [0, 0, 0, 0];
  histogram.forEach((v, ndx) => {
    const ch = ndx % 4;
    max[ch] = Math.max(max[ch], v);
  });
  const scale = max.map(max => Math.max(1 / max, 0.2 * numBins / numEntries));

  const canvas = document.createElement('canvas');
  canvas.width = numBins;
  canvas.height = height;
  document.body.appendChild(canvas);
  const ctx = canvas.getContext('2d');

  const colors = [
    'rgb(255, 0, 0)',
    'rgb(0, 255, 0)',
    'rgb(0, 0, 255)',
    'rgb(255, 255, 255)',
  ];

  ctx.globalCompositeOperation = 'screen';

  for (let x = 0; x < numBins; ++x) {
    const offset = x * 4;
    for (const ch of channels) {
      const v = histogram[offset + ch] * scale[ch] * height;
      ctx.fillStyle = colors[ch];
      ctx.fillRect(x, height - v, 1, v);
    }
  }
}

function showImageBitmap(imageBitmap) {
  const canvas = document.createElement('canvas');

  // we have to see the canvas size because of a firefox bug
  // https://bugzilla.mozilla.org/show_bug.cgi?id=1850871
  canvas.width = imageBitmap.width;
  canvas.height = imageBitmap.height;

  const bm = canvas.getContext('bitmaprenderer');
  bm.transferFromImageBitmap(imageBitmap);
  document.body.appendChild(canvas);
}

function fail(msg) {
  // eslint-disable-next-line no-alert
  alert(msg);
}

main();
  </script>
</html>
